1. Field of the Invention
The present invention relates a cellular radio communication system such as a portable telephone and a mobile telephone, and particularly to a transmission method of control channel information. More particularly, this invention relates to a radio communication system using a code division multiple access (CDMA) system.
2. Description of the Related Art
In the cellular radio communication system, there have hitherto been proposed methods of increasing a subscriber's capacity. A fixed sector system is known as one of such proposed methods. FIG. 1 schematically shows a method of transmitting radio waves in a radio communication system of a fixed sector system.
According to the fixed sector system, a cell is divided into several sectors by a plurality of antennas each having a directional antenna pattern, thereby decreasing an interference to other sector, thus to increase a subscriber's capacity. FIG. 1 shows an example of the manner in which three sectors comprise one sector, i.e. antennas 17a, 17b, 17c each having a fixed directional antenna pattern divide a cell 19 to provide sectors 22, 23, 24.
There will be described the manner in which a cell is divided into sectors in a code division multiple access (CDMA) system that recently receives a remarkable attention as a radio communication multiple access system. When the cell of the CDMA system is divided into sectors, a pilot signal, which is assigned a different phase of spreading code for every sector, is transmitted. Thus, a terminal is able to recognize sectors as different base stations.
In the United States of America, there is known the TIA/EIA/IS-95-A as the standards of the cellular radio communication system of the DS/CDMA (Direct Sequence/Code Division Multiple Access) system. On the other hand, in Japan, the cellular radio system of the CDMA system has been considered by the Association of Radio Industries and Businesses (ARIB).
However, when a cell is divided into sectors, there is then presented the following problems:
The terminal searches other connection channel even during a communication by a so-called cell-search which is a procedure for establishing a synchronization between the terminal and the base station and searching the nearest cell. If the terminal discovers a nominated connection channel which satisfies a predetermined threshold value, then the terminal issues a traffic channel switching request (handoff request) for a different sector or a different cell to the base station. When the cell is divided into the sectors, it is unavoidable that an amount of handoff to be processed between the sectors or the cells increases.
In order to support a softer handoff which is a seamless handoff between the sectors, upon switching, one terminal has to be connected to two sectors, i.e. two base stations simultaneously. However, when two sectors are connected to one terminal at the same time, the terminal transmits and receives control information between the antennas of the two sectors, thereby unavoidably increasing power radiation on the whole. Moreover, upon handoff, auxiliary channels have to be prepared in order to prevent a communication between the sector and the terminal from being disconnected. Thus, when the number of sectors increases, even if the number of sectors is increased, a subscriber's capacity does not increase in proportion to the increase of the number of the sectors.
As a system in which the above-mentioned fixed sector system is further developed and in which there are provided a number of sectors in which a directivity of an antenna is partly overlapped, there has been proposed an adaptive array antenna in which an antenna pattern can be controlled freely and in which an interference from other station can be minimized. This adaptive array antenna was disclosed at the 1995 Society Meeting SB-1-3 of the Institute of Electronics, Information and Communication Engineers. The array antenna is able to decrease an interference by changing a directional beam relative to radio communication terminals located in a variety of directions. Also in the array antenna, a digital beam forming (DBF) which is a beam space expansion is the most effective method because it is easy to control and the digital beam forming is developed owing to the innovation of a digital circuit technology.
In the fixed sector system, there exists a region in which an antenna gain is lowered at the boundary area between the sectors. FIG. 2 illustrates such a situation. As shown in FIG. 2, a base station 20 transmits a control signal by a directional antenna (see sectors 55, 56, 57). When the terminals are located at the boundary area between the sectors, the lowered antenna gain has to be compensated by power control. A control signal, which is generally represented by a pilot signal, needs a large transmit power because it has to be transmitted up to the boundary area of the cell. Unavoidably, this transmit power has to be increased much more.
On the other hand, it is difficult for the system using the array antenna to simultaneously transmit common information such as control channel information to terminals within a cell (sector). The reason for this is as follows. Since there exist a number of antenna elements, conversely, it is difficult to transmit radio waves by an omnidirectional antenna. Therefore, a method of effectively transmitting common information such as control channel information raises a serious problem.
Furthermore, since the pilot signal, which is required by the CDMA system, occupies a large ratio in down-link transmit power, if this mean power is reduced, then it is possible to increase a subscriber's capacity considerably.